1. Field of the Invention
This invention relates to ultrahigh molecular weight polyethylene and more particularly to ultrahigh molecular weight polyethylene molding powders having improved melt processing characteristics.
2. Description of the Prior Art
Ultrahigh molecular weight polyethylene (UHMWPE) may be defined as polyethylene having a molecular weight in excess of about a million and commonly in excess of 2 or 3 million. These extremely high molecular weight polyethylenes provide a number of technically important properties including notched impact strength, energy absorption capacity at high loading rates, tensile impact strength at elevated temperature, resistance to stress cracking and extremely low embittlement temperatures. Necessarily at these high molecular weights, the polyethylene has a high solution viscosity and pronounced viscoelastic characteristics in the melt. In fact, above its crystalline melting point, UHMWPE is not a liquid but behaves much like a rubbery solid. The very high molding pressures needed to form shapes from UHMWPE practically limits the equipment upon which it can be processed to ram extruders and compression molding machines.
In an attempt to make UHMWPE processable at lower temperatures an intermediate molecular weight (e.g. 400,000 to 600,000) polyethylene that behaves more like a liquid above its melting point has been mixed with UHMWPE. When the mixture is heated to above the melting point of the intermediate molecular weight polyethylene, a pumpable slurry-like material results in which the ultrahigh molecular weight polyethylene molding powders are suspended in the liquid phase of the intermediate weight polyethylene. The mixture is processable in conventional injection molding machines and screws extruders in making UHMWPE shapes.
The use of an intermediate molecular weight polyethylene to improve the melt processability of UHMWPE is not without an attendant disadvantage since effective amounts of intermediate molecular weight polyethylene causes a marked decrease in some of the most desirable properties of the UHMWPE, such as impact strength and abrasion resistance. For this reason, the addition of intermediate molecular weight polyethylenes to UHMWPE does not, in itself, present a practical way to improve the melt processability of UHMWPE.